android: downloads on Wi-Fi only

One switch the desktop never needed, off by default: on mobile data every
Save the phone can start — a record, a track, a folder, a file, a
playlist's bulk, the now-playing Save — waits, with the reason on the bar;
streaming is never held back. The walk flips the switch, makes the
connection cellular and sees a download refused.

src/mobile/network.svelte.js

@@ -0,0 +1,22 @@
+/* The phone's connection — for the one guard the desktop never needed:
+   downloads kept to Wi-Fi when the switch says so (You → Downloads).
+   Streaming is never refused: that is what a phone on the network is for.
+   navigator.connection is Chrome's; a WebView without it reports nothing,
+   and nothing is refused. */
+import { settings } from "./prefs.svelte.js";
+import { player } from "./player.svelte.js";
+
+export const cellular = () => {
+    const c = typeof navigator !== "undefined" ? navigator.connection : null;
+    return !!c && c.type === "cellular";
+};
+export const NO_WIFI = "Downloads wait for Wi-Fi — the switch is under You → Downloads.";
+
+/** true when a download may start now; otherwise the reason is on the bar */
+export function downloadsAllowed() {
+    if (settings.wifiOnly && cellular()) {
+        player.error = NO_WIFI;
+        return false;
+    }
+    return true;
+}